The present invention relates to a low-shrinkage polyester film exhibiting an extremely low shrinkage in the machine direction, free from defects such as wave, wrinkle and loosing, and excellent in flatness. More particularly, the present invention relates to a polyester film which is useful as a circuit board for a membrane switch, a flexible printed circuit board, etc. and as an electric insulator for a heat connector, flyback transformer, etc.
A biaxially stretched polyester film is excellent in heat resistance, mechanical strength and chemical resistance, and has been used in various fields including magnetic recording media.
In the utilities of biaxially stretched polyester film, especially in the utilities as electric insulators, substrate for floppy disks, substrate for vertical magnetic recording media, substrate for liquid crystal panel, circuit boards for membrane switches, etc. the film has been required to be low in shrinkage in both the machine and transverse directions so as to avoid deformation due to heat and moisture during the course of production or practical use of the film.
The present inventors have found as a result of the continuous investigation that a film simultaneously satisfying the specific conditions meets with the above purpose. The present invention has been accomplished based on this findings.